The present invention relates to position control devices, such as devices used for the control of the position of the cursor of a CRT display. In particular, the invention relates to adapter means for use with a trackball-type position control device.
Position control devices are used for cursor control in a wide variety of CRT display applications. One such application is in video games. For purposes of this application a "cursor" may be any CRT display pattern which is movable as a unit about the CRT screen. In general, two fundamental types of position control devices are used for this purpose, viz., trackball-type devices and joystick-type devices.
Trackballs and joysticks are typically incompatible in that they produce different types of output signals. Joysticks are of two types, viz., the switch type and the proportional type. Switch type joysticks provide only direction information, by producing a DC level for each of several switch contacts, respectively corresponding to different positions of the joystick, e.g., UP, DOWN, LEFT, and RIGHT. In applications where a switch-type joystick is used, a cursor will continue to move in the direction in which the joystick is held, for as long as it is held in that direction, and it will move at a predetermined rate under program control of the host processor in the associated video game.
A proportional-type joystick produces both speed and direction information and causes the cursor position to follow the position of the joystick. Thus, if the joystick is moved from its center position to its full UP position, the cursor will undergo a similar movement and will move at a rate which is proportional to the rate at which the joystick is moved. This type of joystick essentially produces two varying voltage levels which respectively correspond to the horizontal and vertical axes and vary in direct response to the components of movement of the joystick along these axes.
A trackball device also produces speed and direction information, but it does so in a fundamentally different way from a proportional joystick. The trackball device output consists of four pulse trains, a set of two for each of two orthogonal axes. Thus, components of rotation of the trackball about one axis generates one set of two pulse trains and components of rotation about the other axis generate another set of two pulse trains. The pulse repetition rates are directly proportional to the speed of rotation of the trackball about the corresponding axis, and the phase relationship of the two pulse trains in each set is related to the direction of rotation of the trackball about the corresponding axis.
Because of the fundamentally different types of outputs produced by these different types of position control devices, a video game which is designed for use with a joystick type position control device cannot be used with a trackball type position control device. This is a significant disadvantage, because trackball devices afford significantly more accurate and realistic position control than is afforded by joysticks, particularly as regards control of the speed with which the cursor moves. Furthermore, video games which use switch-type joystick inputs are not intended to accept speed information at all, since the cursor is intended always to move at the same speed as determined by program control.
Another drawback of existing devices is that the control circuits for trackball devices typically utilize from four to six separate integrated circuit chips and, therefore, are of relatively complex and expensive construction.